This application claims the benefit of Korean Application No. 99-52388, filed Nov. 24, 2000, in the Korean Patent Office, the disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a phase change optical disc having a recoding layer that changes phases according to an incident beam, and more particularly, to a phase change optical disc in which the structure of a dielectric layer disposed between a substrate and a recoding layer is improved.
2. Description of the Related Art
Phase change optical discs are widely known as optical discs which enable recording/reproduction of information by using an optical feature in which a portion of the recording layer which receives an incident laser beam is converted to a crystal state or an amorphous state according to a power of the incident laser beam.
FIG. 1 shows a conventional phase change optical disc. The phase change optical disc 10 includes a substrate 11, formed of a transparent material such as polycarbonate, a first dielectric layer 12, a recording layer 13, a second dielectric layer 14, a reflective layer 15 and a protective layer 16 which are formed on the substrate 11 in order.
The first and second dielectric layers 12 and 14 are layers which protect the substrate 11 and the reflective layer 15 where the temperature of the recording layer 13 is quickly changed by an incident laser beam input to the substrate 11 in a direction indicated by an arrow A. The first and second dielectric layers 12 and 14 are formed of ZnSxe2x80x94SiO2, a material having a superior thermal resistance feature. The reflective layer 15 is formed of a material having a superior reflectance such as aluminum (Al), to reflect an incident beam. The protective layer 16 is formed of an ultraviolet cure resin layer and the protective layer 16 protects the first and second dielectric layers 12 and 14, the recording layer 13 and the substrate 11. The recording layer 13 is formed of a material such as an Sbxe2x80x94Texe2x80x94Inxe2x80x94Se based compound and the phase of a portion to which a laser beam is emitted may be changed according to the power of the incident laser beam. For example, during recording, a high power laser beam is emitted to the recording layer 13. Accordingly, the portion of the recording layer 13 to which the laser beam is emitted melts and then is cooled to be converted to an amorphous state, forming an information mark. The information mark in an amorphous state and a neighboring portion in a crystal state are different in optical reflectance so that information is recorded/reproduced due to the difference in the optical reflectance. For erasing recorded information, a laser beam having a power of about ⅓-xc2xd of the power which is used for recording is emitted. The information mark portion is heated by the laser beam to a temperature higher than a crystallization temperature and simultaneously lower than a melting temperature, and then the information mark portion is cooled down. Accordingly, the information mark in an amorphous state is crystallized again so that the information is erased.
To obtain a stable reproduction signal, the difference between the reflectance Rc of the crystal recording state and the reflectance Ra of the amorphous recording state, that is, Rcxe2x88x92Ra, is preferably 10% or more. Also, where overwrite information is recorded on a surface on which information has already been written, an error in a signal can be reduced only where an information mark having a desired size is recorded regardless of the state of the recording layer 13. Accordingly, the ratio between the optical absorptivity AC of the crystal recording state with respect to the optical absorptivity Aa of the amorphous recording state, that is, the optical absorptivity ratio, Ac/Aa, should be at least 1.2.
For the phase change optical disc, to increase the recording density by decreasing the interval between tracks, a study of using a blue laser beam having a short wavelength less than 450 nm instead of near infrared rays or a red laser beam has recently been performed. Where a blue laser beam having a short wavelength is used, a diameter of an optical spot and an interval between tracks are reduced so that the recording density increases accordingly. Where the interval between tracks decreases, a cross-erase phenomenon in which an information mark recorded on a neighboring track is erased during recording is serious. The cross-erase phenomenon is reduced as the optical absorptivity ratio Ac/Aa increases. Thus, to increase the recording density, the optical absorptivity ratio Ac/Aa needs to be increased to a maximum degree.
However, in the above-described phase change optical disc, as it is well known, a light wave reflected by the first dielectric layer and a light wave reflected by the crystal recording state generate destructive interference. Accordingly, in the crystal recording state, the reflectance decreases and the optical absorptivity increases while, in the amorphous recording state, the reflectance increases and the optical absorptivity decreases. Thus, although a high optical absorptivity ratio can be maintained, it does not exceed 1.3.
To solve the above problems, it is an object of the present invention to provide a phase change optical disc in which the structure of the dielectric layer between the substrate and the recording layer is improved to increase the optical absorptivity ratio so that the cross-erase phenomenon is reduced.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
Accordingly, to achieve the above object, there is provided a phase change optical disc comprising a substrate, a first dielectric layer formed on the substrate, a recording layer formed on the first dielectric layer which recording layer is convertible into one of an amorphous state and a crystal state according to a power of an incident beam, a second dielectric layer formed on the recording layer, and a reflective layer formed on the second dielectric layer. In a phase change optical disc according to the present invention, the first dielectric layer is formed of a material having a refractive index of 2.7 or more with respect to the incident beam.